This invention relates to an optical switching unit, particularly for switching from faulty/transmission facilities to standby transmission facilities in optical transmission systems. Such a transmission facility may be a transmit laser, a receiver photodiode, or an optical fiber, for example.
In optical transmission systems, optical fibers interconnect a transmitter and a receiver. Because of the wide transmission bandwidths of optical fibers, a single such fiber generally suffices to transmit the usual amounts of data. To be able to continue to transmit data if the optical fiber should be damaged, one or more spare fibers are laid parallel to that optical fiber. If any damage to the optical fiber is detected, a control unit will cause the transmitter and receiver to switch to a spare fiber.
To increase the reliability of optical transmission systems, a standby laser and/or a standby photodiode, which are used in case of failure of the main device, are frequently incorporated into the system. Since communication is interrupted while changeover to a standby transmission facility is effected, the dead time of the transmission system, i.e., the time required for the changeover, should be as short as possible.
European Patent Application EP-A1-519 712 discloses an optical bus network in which the two end nodes of the bus are connected by optical fibers to an additional monitoring node (xe2x80x9cterminal nodexe2x80x9d). The monitoring node, which is thus connected directly between the two end nodes, establishes a direct connection between the two end nodes with the aid of a switch if there is a discontinuity in the bus. As a result, the nodes in the immediate vicinity of the discontinuity become the new end nodes of the bus. To be able to detect discontinuities (e.g., a break in a fiber), the monitoring node includes optical-to-electrical transducers with which it monitors the signals from the end nodes.
Offenlegungsschrift DE-A1-44 33 031 discloses a method of switching to one or more spare lines in unidirectional or bidirectional optical transmission systems (see FIG. 6). If the receiver portion of a transceiver unit SE6A at one end of a bidirectional optical transmission system does not receive the data stream transmitted over an optical fiber FA6 or receives the data stream only in greatly disturbed form, it will communicate this to a control unit ST6A associated with this end. The control unit causes the transmitter and receiver at this end to be switched to a spare line FAE6. The receiver portion of a transceiver unit SE6B at the other end of the transmission link then determines that no signal is now present on the originally used optical fiber FA6. In response to this, the control unit ST6B initiates a changeover to the spare line FAE6 at that end of the transmission link as well.
Under certain circumstances, for example in the case of very long transmission links, the probability that not only the optical fiber actually intended for transmission but also the spare fiber will be damaged is relatively high. In that case, the data transmission will collapse. To prevent this, more than one spare fiber may be laid, as is known from-the above-cited DE-A1-44 33 031. Particularly in the case of long transmission links, however, the additional optical fibers to be laid represent a considerable cost factor.
It is therefore an object of the invention to provide an optical switching unit with which changeover can be effected from faulty transmission facilities to standby transmission facilities in optical transmission systems. The optical switching unit is to be simple in construction and universally applicable. In particular, the optical switching unit is to permit the construction of an optical transmission system which allows reliable data transmission even in the event of frequent damage to optical fibers but requires as few spare optical fiber as possible.
The invention attains the object with an optical switching unit for use in optical transmission systems, comprising an nxc3x97m optical space switch having n+m ports, with n=1, 2, 3, 4, . . . and m=2, 3, 4, . . . , wherein means are provided for monitoring the optical signals arriving at at least one port on each side of the optical space switch and for controlling the position of the nxc3x97m space switch in accordance with changes in the optical signals being monitored. One, a number, or all of the ports on both sides of an nxc3x97m optical space switch are monitored by photodiodes which are connected to a control unit incorporated into the optical switching unit. The control unit controls the position of the optical space switch in accordance with changes in the signals applied at the ports. Depending on how the control unit is programmed, the optical switching unit according to the invention can be used to advantage for different functions.
In a first embodiment, the optical switching unit is used as a switch in an optical transmission link. The optical link consists of a first optical fiber and a parallel, second optical fiber as a spare fiber. Optical switching units according to the invention are integrated into the link at given intervals. If the first optical fiber is broken, the optical switching units adjacent to the break will detect this and switch to the existing spare fiber. Since the switching processes are initiated and controlled in the transmission link itself and not centrally at the transmitter end, the dead time of the transmission system, i.e., the time in which no data transmission takes place, is very short.
The division of the transmission link into several sections ensures reliable data transmission even if both optical fibers have one or even more breaks since, according to the invention, only the respective faulty section is bypassed by the spare fiber. Another advantage is that the positions of the optical switches can also be controlled from the transmitter directly via the optical fibers if the optical switching unit is programmed in such a way as to detect particular patterns in the data stream and initiate corresponding switching processes in response thereto.
In a second embodiment, the optical switching unit is used in an arrangement of transmit lasers to switch to a standby laser if one of the transmit lasers fails. In that case, too, the transmission capability of the system can be restored in the shortest time.
In a third embodiment, the optical switching unit is used as an optical selector switch. In packet-switched transmission systems, the optical switching unit is capable of detecting the address field in the packet header and of establishing connections in accordance with this information. Thus, the switching unit according to the invention can be used to advantage in future optical ATM exchanges, for example.